Excellent advice above!

The fans do have a permanent live, so that is why they stay on without ignition.

You certainly should rewire, the fan loom and the switch was sub standard to start leave alone with the age now, the switch brings the fans on to early and for to long a duration thanks to the massive 9 degree span, which is what leads to the melting fuses found in so many of these! Again the "Uprated" switch can help here.

Furthermore the fuel pump circuit suffers from the old wiring as it builds resistance and the pump cannot get the power it needs when under load, worst case scenario is a meltdown! So again ideally this needs the voltage checking UNDER load and ideally rewiring, if nothing else it alleviates the load from the fuse box.

What is your base idle when at running temperature? As said the engine speed drops due to the demand on the electrical supply from the fans, but if it's to low it may vibrate etc.

Martin